Worm drive for electrically wound clock

ABSTRACT

For an electrically wound clock, a worm and worm gear drive, wherein the worm is a space-wound helical wire spring, frictionally fitted on a motor shaft, extending beyond the end of the shaft, and unsupported at its outboard end. Engagement with the worm gear takes place at a point beyond the end of the shaft.

United States Patent [191 Metzinger [451 May 7,1974

[ WORM DRIVE FOR ELECTRICALLY WOUND CLOCK [75] Inventor: Leonard L.Metzinger, Delavan,

Wis.

[73] Assignee: Bunker Ramo Corporation, Oak

Brook, Ill.

[22] Filed: Sept. 29, 1972 [21] Appl. No.: 293,319

[52] US. Cl. 74/425 [51] Int. Cl. Fl6h 1/16 [58] Field of Search74/424.5, 458, 425

[56] References Cited UNITED STATES PATENTS 3,238,804 3/1966 Goodykoontz74/458 X 2,151,191 3/1939 Crane et al. 7'4/458 X 3,163,054 12/1964Werner..... 74/458 X 3,049,936 8/1962 Schnell 74/425 X PrimaryExaminer-Leonard H. Gerin Attorney, Agent, or Firm-D. R. Bair; F. M.Arbuckle [5 7] ABSTRACT For an electrically wound clock, a worm and wormgear drive, wherein the worm is a space-wound helical wire spring,frictionally fitted on a motor shaft, extending beyond the end of theshaft, and unsupported at its outboard end. Engagement with the wormgear takes place at a point beyond the end of the shaft.

5 Claims, 2 Drawing Figures WORM DRIVE FOR ELECTRICALLY WOUND CLOCKBACKGROUND OF THE INVENTION ciency of the drive system is of greatimportance, be-

cause of its effect on battery life. A battery powered clock which isout of service frequently because the batteries are exhausted can be thecause of much user dissatisfaction. Hence it is important that the drivesystem waste a minimum of energy in friction.

Further, in a small, mass-marketed device for general public use, whereprice competition is strong, every effort must be madeto save fractionsof pennies in the design and manufacture of the device.

In certain types of battery powered clocks, a tiny motor is used to windthe mainspring at uniform intervals, and a motor of suitably small sizefor use in a clock must run at rather high speed to have sufficientpower output. This dictates the use of a worm and worm gear as a speedreduction means between the motor and the spring arbor. In prior artclocks it has been customary to use a molded or machined metal orplastic worm engaging the circumferential worm wheel teeth. In thepresent state of the art, the cut or molded worm must be held to closetolerances for diameter, pitch of teeth, smoothness, and concentricityor'run-out. If these pa rameters are not carefully maintained, or if, inthe case of a molded part, there is flash or irregularity at a partingline, the worm and wheel combination. may generate a considerable amountof noise, with the objectionable results-which have been described. Ithas been found extremely difficult to control the dimensional propertiesand the parting line roughness of a molded plastic worm to an extentsuch that the noise made by the system in the periodic winding cyclewould not be objectionable in a quiet room.

Worms using a spring to minimize operating noise have been known, asseen in US. Pat. Nos. 2,682,176 and 3,268,268, but they have required anarbor for backup support, which must be manufactured by an expensivetype of machining process. They have also required bearings at each endof the worm, an undesir ably costly and complicated construction for atimpiece.

SUMMARY OF THE INVENTION This invention involves the provision of a wormconsisting of a helical wire spring, tightly fitted onto the drivingshaft, extending beyond the end of the shaft, unsupported at itsoutboard end, and engaging a worm gear at a point beyond the end of theshaft. in the small sizes involved in battery-operated clocks, thespring can be assembled to the shaft with the aid of simple fixtures, oreven by hand, the inherent resilience of the spring serving to retain iton the driving shaft. No special machining or grooving of the drivingshaft is required.

The pitch of the spring is made the same as that of the worm gear withwhich it meshes; this gives a quiet, long-wearing low friction drive,particularly suitable for service in a clock mechanism. The spring canbe simply and economically fabricated on automatic spring coilingmachines, with no initial investment such as the cost of molds for aplastic worm.

BRIEF DESCRIPTION OF THE DRAWING To facilitate further explanation of aconstruction having these advantages, the accompanying drawings areprovided, wherein:

FIG. 1 is an elevational view of a drive construction embodying myinvention, and

FIG. 2 is a cross sectional view taken on the line 22 of FIG. 1, andpartly broken away, illustrating the feature of the construction bywhich the worm is retained on the driving shaft.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the accompanying drawings,the motor 10 has an output shaft 12, over which is fitted a straighthelical open wound coil spring 14, a substantial portion of whichextends beyond the outer end 16 of the shaft 12. The portion 18 of thespring which thus extends beyond the shaft engages with the teeth 20 ofa worm gear 22, which is mounted for rotation on a shaft 24, being partof the winding mechanism of a clock, not otherwise shown. As will belater explained, the spring is a friction fit on the shaft 12, andtherefore when the motor is energized and the shaft 12 rotates, thespring is rotated about its axis, and serves asa worm to drive the wormwheel.

This construction has a number of advantages over the conventional wormgear drive. A suitable spring can be manufactured much more cheaply thana machined worm. Where a material such as bright music wire is used forthe spring, a very smooth surface is presented, which contributes toquiet operation and has long-wearing characteristics.

The arrangement 'shown in the drawing, wherein the working portion ofthe spring extends beyond the end of the driving shaft as a cantilever,provides a degree of resilience which permits a considerable degree ofmanufacturing tolerance in the distance and alignment between the axisof the driving shaft 12 and the axis of the driven shaft 24.

The spring may be mounted on the driving shaft 12 with a minimum ofpreparation. No grooving, threading or special preparation of the shaftis required, except that a chamfer 26 may be provided to assure thatthere is no burr at the end of the shaft, and to aid in centering andleading the spring when it is pressed onto the shaft.

The spring as originally manufactured can be straight, i.e., of uniformdiameter throughout. The internal diameter of the spring relative to thediameter of the shaft 12 must be controlled to provide a slightinterference fit. That is, the coils of the spring which are slippedonto the shaft must experience some expansion in internal diameterbeyond the normal internal diameter of the spring. This is illustrated,considerably exaggerated, in FIG. 2, which shows a cross section throughthe driving shaft 12 looking toward the spring. The numeral 28designates a coil of the spring surrounding the shaft. A portion of thefigure is broken away, to reveal another coil of the spring, such as 30,one of those in the portion 18 beyond the end of the shaft. Both figuresof the drawing show, in exaggerated fashion, that the coil 30 is ofsmaller diameter than the coil 28. The spring 14 is thus retained on theshaft 12 by its own inherent resilience, which is adequate to keep it inplace, meeting the torque requirements of a clock wind drive such as hasbeen described, or of similar light duty applications.

When an open wound helical spring is compressed, its internal diameterincreases. This effect may be sufficient, in most applications, topermit the spring to be mounted on the motor shaft by the application ofan axial force, either in a simple jig, or by hand. It is compressedagainst the end of the shaft, and may simultaneously be twisted in adirection tending to unwind it. Both actions tend to cause the coils toincrease somewhat in diameter, and they are thus enabled to slip ontothe shaft. After the axial and twisting forces are removed, and coilstend to return to their normal uncompressed pitch and diameter, thusgripping the shaft.

While there are some teachings in the patent literature that a wire wormcan advantageously be used having a pitch differing from that of theworm gear with which it is operated, that has been found not to be thecase for a clock-wind drive. Experiments were made with a clock windmechanism in which a 36-tooth 48 pitch worm gear was employed on thewinding arbor. The correct pitch for such a gear is 0.0654 inch, or 1.66

mm. The wire worm was made of bright music wire,

Average Duration Worm pitch Motor current Wind cycle Ampere-hoursmillimeters milliamperes seconds per year L66 I08 1.] 2.l7 l.52 208 L76.48 [.78 I28 l.2 2.8l

The last column shows the computed values of ampere-hours per yearrequired for the total number of winds in that time, for each of theworm-gear combinations listed. it is obvious that the nominal pitch of1.66 mm. provides the conditions for the lowest battery consumption andthat the spring should be sufficiently stiff to maintain this pitchthroughout the range of torque encountered in the wind operation.

Observations of operating noise were also made. The frequency componentsof audible sound were somewhat different in the test of 1.52 mm. pitch,attributable to the fact that friction loading of the worm gearingslowed down the motor considerably, as shown in the table by theincreased cycle time. Otherwise, however, there was no significantdifference, and the sound levels for the various trials were about thesame. They were noticeably lower, however, than the noise levels ofprevious similar devices using moulded plastic and solid metal worms.

The simplicity, economy, and operating advantages of the constructiondescribed will be noted by those skilled in the art. Some changes may bemade in the application, or in the details of construction, withoutdeparting from the basic concept of the invention, which is defined inthe following claims.

I claim:

1. Drive apparatus for winding a spring in an electrically-poweredclock, operative to wind said spring in short wind cycles, said windcycles being separated by relatively long non-operative periods, whereinpower is transmitted from a rotatable shaft to a worm gear, andcharacterized by a worm comprising a helical wire spring secured to saidshaft coaxially therewith by the gripping of several turns of saidspring on said shaft, said spring extending beyond the end of saidshaft, and engaging said worm gear at a point beyond the end of saidshaft, said spring having a pitch the same as that of said worm gear,and being sufficiently rigid axially so that the pitch of the springdoes not vary with the varying torque imposed on said spring by saidworm gear during said wind cycle,and the number of teeth of said wormgear embraced between adjacent turns of the spring remains constant.

2. In a drive apparatus for an electric clock wherein power istransmitted from a rotatable shaft to a worm gear, a worm comprising anopen-wound helical wire spring of normal internal diameter less than thediameter of said shaft, said spring having a portion of its lengthsurrounding said shaft and retained thereon by its inherent elasticity,and having another portion of its length extending beyond the end ofsaid shaft, said spring engaging the worm gear in driving relationshipat a point beyond the end of said shaft.

3. Means for transmitting energy from an intermittently operated clockmotor having an output shaft, to a worm gear, comprising a worm in theform of an open-wound helical wire spring of normal internal diameterless-than the diameter of said shaft, the spacing between turns of saidspring being sufficient so that when the spring is compressed, itsinternal diameter increased enough to permit the turns so compressed topass onto said shaft, the turns gripping the shaft when compression isreleased and thereby retaining the spring in driving engagement with theshaft, said spring when so retained having a portion including aplurality of turns extending beyond the end of the shaft, and saidportion operably engaging the worm gear.

4. The invention in accordance with claim 3, wherein said spring issufficiently rigid axially so that the pitch of the spring does not varywith the varying torque imposed on said motor throughout itsintermittent operation, and the number of teeth of said worm gearembraced between adjacent turns of the spring remains constant.

5. Drive apparatus intermittently operable to wind a clock spring,comprising a motor having an output shaft, a worm in the form of anopen-wound helical wire spring having one end secured to said shaft andanother end extending beyond said shaft and operatively engaging a womigear, whereby said spring is laterally resilient at the point of contactwith said worm gear; but sufficiently rigid axially that the pitch ofthe spring does not vary with the amount of variation in loadimoperation.

1. Drive apparatus for winding a spring in an electricallypowered clock,operative to wind said spring in short wind cycles, said wind cyclesbeing separated by relatively long nonoperative periods, wherein poweris transmitted from a rotatable shaft to a worm gear, and characterizedby a worm comprising a helical wire spring secured to said shaftcoaxially therewith by the gripping of several turns of said spring onsaid shaft, said spring extending beyond the end of said shaft, andengaging said worm gear at a point beyond the end of said shaft, saidspring having a pitch the same as that of said worm gear, and beingsufficiently rigid axially so that the pitch of the spring does not varywith the varying torque imposed on said spring by said worm gear duringsaid wind cycle, and the number of teeth of said worm gear embracedbetween adjacent turns of the spring remains constant.
 2. In a driveapparatus for an electric clock wherein power is transmitted from arotatable shaft to a worm gear, a worm comprising an open-wound helicalwire spring of normal internal diameter less than the diameter of saidshaft, said spring having a portion of its length surrounding said shaftand retained thereon by its inherent elasticity, and having anotherportion of its length extending beyond the end of said shaft, saidspring engaging the worm gear in driving relationship at a point beyondthe end of said shaft.
 3. Means for transmitting energy from anintermittently operated clock motor having an output shaft, to a wormgear, comprising a worm in the form of an open-wound helical wire springof normal internal diameter less than the diameter of said shaft, thespacing between turns of said spring being sufficient so that when thespring is compressed, its internal diameter increased enough to permitthe turns so compressed to pass onto said shaft, the turns gripping theshaft when compression is released and thereby retaining the spring indriving engagement with the shaft, said spring when so retained having aportion including a plurality of turns extending beyond the end of theshaft, and said portion operably engaging the worm gear.
 4. Theinvention in accordance with claim 3, wherein said spring issufficiently rigid axially so that the pitch of the spring does not varywith the varying torque imposed on said motor throughout itsintermittent operation, and the number of teeth of said worm gearembraced between adjacent turns of the spring remains constant.
 5. Driveapparatus intermittently operable to wind a clock spring, comprising amotor having an output shaft, a worm in the form of an open-woundhelical wire spring having one end secured to said shaft and another endextending beyond said shaft and operatively engaging a worm gear,whereby said spring is laterally resilient at the point of contact withsaid worm gear; but sufficiently rigid axially that the pitch of thespring does not vary with the amount of variation in load imposed uponit throughout the intermittent winding operation.